As is well known to those skilled in the art, the two most common types of overrunning or one way clutches in use in high speed applications, such as in the gear sets of automatic transmissions, are sprag clutches and roller clutches. While roller clutches have a significant cost advantage, sprag clutches are currently used in many very high speed applications, because sprag elements are, in general, less subject to control problems at very high speeds than are rollers. However, an improved roller clutch that matched or even exceeded the high speed performance of a sprag clutch would be very desirable, because of cost considerations. Another common problem with roller clutches is that the rollers are generally not retained to the cage very strongly for shipping and handling purposes, and can be easily dislodged. Both the roller control problem and roller retention problem will be described in detail, as well as prior art proposals to deal with the problems.
The typical roller clutch has a plurality of cylindrical rollers that are located, after clutch installation, in wedging pockets formed between a cylindrical pathway on one clutch race and a series of sloped cam ramps on the other clutch race, usually called the cam race. A cage fitted to one race usually has side rails of some type to axially confine the ends of the rollers and prevent them from moving axially out of the annular space between the races. Individual energizing springs continually urge each roller up the cam ramp to a ready position. Because of eccentricities between the races and other forces, the rollers will and must move significantly, in the circumferential sense, back and forth within the wedging pockets during clutch operation, and will not all be in the same position at the same time. In fact, a great advantage of the roller clutch, compared to the sprag clutch, is that since the rollers can freely seek their own individual positions, the clutch races need not be maintained rigidly coaxial by outside bearings, as with sprag clutches. This circumferential movement of the rollers is generally referred to as roller travel, and anything in the clutch that would interfere with it would be highly undesirable. In a typical roller clutch, the only structure directly touching the roller to control its position, other than the race surfaces themselves, are the inside surfaces of the side rails, which face the axial ends of the rollers, and the energizing spring. The degree of interference with roller travel caused by the direct bumping and sliding contact of the roller ends with the inside surfaces of the cages side rails is limited. Likewise, the spring is not greatly limited in its travel by its energizing spring, which touches the roller only along one side of the roller, and which does, and must, compress and expand freely. Ideally, the energizing spring is only as strong as is necessary to provide the constant ready position bias. This is so that the roller will not be pushed any more strongly than necessary into the race surfaces. If the roller is pushed too strongly into the race surfaces, the traction of the pathway, during overrun, can spin the rollers against the cams, especially at high speeds, causing cam wear. This is especially a problem in inner cam clutches at high speeds, because the effects of centrifugal force throwing the rollers up the cams and into the pathway during overrun, as well as spring force, add to roller traction and spin wear.
While there is no significant limitation on circumferential roller travel by the cage or spring in a conventional roller clutch, neither is there any particularly effective control of the roller's operation during overrun. The need for free roller travel makes control and retention of the wedging elements much more difficult than it is in sprag clutches. Roller control is desirable, especially at high speeds. Lack of control of the roller spin has already been mentioned. Likewise, roller skew can occur when various external forces overcome the relatively weak energizing spring during overrun and send the roller out of ready position into the wide end of the wedging pocket. Once the roller is out of contact between the pathway and cam ramp, which provide the only real effective control over its orientation, it can skew out of parallel with the race coaxis.
The clutch designer must not only consider the rollers during clutch operation, but also their retention prior to installation, generally called shipping retention. If it were common to ship a roller clutch already installed between its races, then retention of the rollers to the cage would not be a problem. However, roller clutches are generally shipped alone, to be installed between the races later. The roller shipping retention system most often used is simple, but is very limited in the strength with which it can retain the rollers to the cage. Conventionally, during shipping, the only control of the rollers is from the resilience of the springs, which, in an expanded state, press the rollers against V grooves on the cage. The rollers are thus not only retained to the cage per se, but are also maintained in a definite shipping position, circumferentially spaced in a position where they will each slide onto a respective cam ramp when the clutch is installed. A great shortcoming of the system, despite its simplicity and low cost, is that the strength with which the rollers are retained is only as great as the resilience of the springs, that is, how hard the springs press the rollers into the v grooves. And, the springs, as noted above, are not particularly strong, nor should they be.
Few patents speak to the problem of roller control during clutch overrun. One patent that considers the roller spin and wear problem is U.S. Pat. No. 2,044,197 to Barthel. It discloses an inner cam clutch in FIG. 1, where the energizing springs act indirectly on the rollers through intermediate, asymmetrical weights pivoted to the cam race. The heavier side of the weights is located between the pivot and the rollers, so that, when the inner race rotates at high speed during overrun, centrifugal force pivots the weights away from the rollers, counteracting the springs. While this would lessen or remove the contribution to roller spin that the springs make, it would do nothing to counteract the pathway-roller traction that results just from the centrifugal force that throws the rollers up the cam ramps and into the pathway. Furthermore, in an application where there was no centrifugal force on the roller, that is, in the case where the cam race was static and the outer race turned, the weights would fail to pivot at all, and would therefore do nothing to relieve the spring force. In addition, the design would necessitate drastic changes in the size, operation, travel distance and cost of the energizing springs that are conventionally used, and would, therefore, be totally impractical in the context of a typical automatic transmission gear set. Nor does the design appear to do anything to better confine the roller to control or limit its potential skew, as the roller is still contacted on one side only.
As to control of the rollers during shipping and handling, some roller retention systems have been proposed in which the roller retention, while not totally independent of the spring, is at least independent of the resilience of the spring, and thus more secure. One such roller clutch is disclosed in U.S. Pat. No. 3,994,377 to Elmore. There, the energizing springs consist of axially opposed pairs of tabs lanced out from the metal cage side ails, which extend into hollowed out ends of the rollers, and which push or pull on the rollers to energize them. Roller retention would be quite secure, because the spring tabs are short and stiff. However, the clutch would be totally unworkable in many applications. The spring tabs, by their very nature, could only be used with metal cages, whereas plastic cages are often preferred. Steel suitable for cages would make very good or tough springs, and the short tabs would not be able to flex over a very long distance. Besides the inevitable weakening of the rollers from hollowing out their ends, there would be a great limitation in the roller travel possible. The rollers would inherently be able to travel less than their own diameter. Roller travel must often be greater than that during clutch overrun, due to eccentricity between the races and external forces acting on the roller. To be truly practical, the shipping retention scheme should place no limitations on cage material, and should use absolutely conventional rollers and springs, and present no limitations on the operation of either. A clutch that comes very close to that ideal is disclosed in the U.S. patent application Ser. No. 895,143, allowed Mar. 25, 1987, issued Feb. 16, 1988 as U.S. Pat. No. 4,724,940, assigned to the assignee of the current invention. There, shipping retention of the rollers, and of the springs, results from tabs on the sides of the endmost loops of the springs, which are trapped between the rollers on one side and ramps molded to the plastic cage side rails on the other side. This, in turn, holds the rollers against cage stop surfaces, and the net result is that springs, rollers and cage are all cooperatively held together before shipping, strongly and in a proper position for installation. Although the springs are necessary to roller retention, there is no reliance at all on the resilience of the springs, which may be totally uncompressed. When the pathway race is added, which is done by the ringing in or twisting method familiar to those skilled in the art, the tabs slide past the ramps, totally freeing the rollers and springs, which can then operate without limitation. While the retention scheme here is sound and secure, a special spring must be used, and there is nothing to deal with the roller skew problem or roller spin problem.